A low energy fuse of the type referred to was first described in U.S. Pat. No. 3,540,729 and numerous subsequent patents have been published. In broad terms the fuse consists of a narrow plasic tube with a pyrotechnic or self-explosive reactive matter disposed within the tube channel. The amount of reactive material is sufficient to give a high speed shock wave in the channel, able to ignite secondary or functional pyrotechnical devices such as detonators or transmission caps for blasting networks. Yet the amount of reactive material is sufficiently small to confine the reaction within the tube without destroying, disrupting or even deforming it and to make the overall device safe, harmless and noiseless in use.
Although the device is simple in principle, the physical demands placed on it are not. A substantial radial strength is needed to resist the forces produced by the shock. Signal speed is lost, or the wave halted, if the tube is substantially defomed or ruptured. Radial strength is also needed to avoid compression and external damages and to allow crimp attachment of functional devices to the tube. Substantial axial strength with maintained elasticity is needed to take up forces involved in handling, network connection and charging operations. Overall toughness is needed to sustain the harsh field conditions before and during blasting. Further desired properties are suitable friction properties and impermeability to moisture and oil.
The reactive material is typically a powder introduced in the channel. For that reason a unique constraint on the fuse tube is that the interior surface must have suitable powder adhesion properties. A too weak attraction may mobilize the powder, giving signal interuptions due to material rarefactions or clogs. Too stong bonds counteracts rapid reaction and dust explosion.
The fuse tube is produced in considerable lengths and the tube materials must be inexpensive and the manufacturing methods cost effective.
The demands are partly contradictory and single-layered tubes tend to require a compromise between desired properties. It has been suggested in U.S. Pat. No. 4,328,753 to make a two-layer tube and select inner and outer materials of different properties but the materials are not optimally used solely thereby.
It has been suggested, in for example Canadian patent 1 200 718 and U.S. Pat. No. 4,817,673 to increase axial strength by incorporating longitudinal reinforcing filaments in the tube material. The resulting inelastic tube is unable to absorb elongation under field conditions and tend to break or disengage from its detonator when subjected to strain. The tube material is not efficiently utilized in spite of the increased costs for manufacture and reinforcements.
The U.S. Pat. No. 4,607,572 describes a manufacturing method in which an inner tube with suitable adhesive properties is first manufactuted and then elongated under overextrusion to increase manufacturing speed, minimize the amount of adhesive inner material and give an orientation to the elongated tube material. The respective materials are not efficiently used as orientation is concentrated to the inner layer, resulting in radial brittleness, whereas the outer layer contributes little to axial strength. A product inclined to fracture will result unless stretching is limited.
The European patent specification 327 219 describes a single-layer tube extruded from a mixture of a draw orientable polymer and a minor amount of a polymer of adhesive quality. In manufacture the adhesive polymer is said to concentrate at the inner surface of the tube and substantial orientation of the polymers is imposed in a cold stretching step following extrusion. The orientation adds substantial axial strength to the tube but the radial strength is lost proportionally, again resulting in a poor resistance to the shock and a poor utilization of the inherent strength capacities of the polymers is used.
These more advanced tube designs add to production costs and problems. Simple coextrusion or overextrusion can be done fairly easily but do not utilize the full strength of the materials. Orientation by substantial stretching can he run efficiently but tend to give unacceptable radial properties. Limited stretching may be preferred but tends to give unstable process conditions and final tube properties unless the draw orientable polymer is supported by other layers or conditions.